MR-guided stereotactic neurosurgery--comparison of fiducial-based and anatomical landmark transformation approaches.

For application in magnetic resonance (MR) guided stereotactic neurosurgery, two methods for transformation of MR-image coordinates in stereotactic, frame-based coordinates exist: the direct stereotactic fiducial-based transformation method and the indirect anatomical landmark method. In contrast to direct stereotactic MR transformation, indirect transformation is based on anatomical landmark coregistration of stereotactic computerized tomography and non-stereotactic MR images. In a patient study, both transformation methods have been investigated with visual inspection and mutual information analysis. Comparison was done for our standard imaging protocol, including t2-weighted spin-echo as well as contrast enhanced t1-weighted gradient-echo imaging. For t2-weighted spin-echo imaging, both methods showed almost similar and satisfying performance with a small, but significant advantage for fiducial-based transformation. In contrast, for t1-weighted gradient-echo imaging with more geometric distortions due to field inhomogenities and gradient nonlinearity than t2-weighted spin-echo imaging, mainly caused by a reduced bandwidth per pixel, anatomical landmark transformation delivered markedly better results. Here, fiducial-based transformation yielded results which are intolerable for stereotactic neurosurgery. Mean Euclidian distances between both transformation methods were 0.96 mm for t2-weighted spin-echo and 1.67 mm for t1-weighted gradient-echo imaging. Maximum deviations were 1.72 mm and 3.06 mm, respectively.

Reducing the radiation dose during excretory urography: flat-panel silicon x-ray detector versus computed radiography.

OBJECTIVE: The purpose of the study was to examine the possibilities for reducing radiation exposure in uroradiology using digital flat-panel silicon X-ray detector radiography. We compared the subjectively determined image quality of abdominal radiographs and urograms obtained on a digital flat-panel detector radiography system with those obtained on a computed radiography system. SUBJECTS AND METHODS. Fifty patients who had a clinical indication for urography underwent unenhanced abdominal imaging that was alternately performed using flat-panel silicon X-ray detector radiography or computed radiography. For patients who required a second radiograph with contrast medium, the examination modality was changed to avoid exposing the patients to excess radiation. The images obtained on flat-panel X-ray detector radiography were obtained at half the radiation dose of the images obtained on computed radiography (800 speed vs 400 speed). The resulting 50 pairs of images were interpreted by four independent observers who rated the detectability of structures of bone and the efferent urinary tract relevant to diagnosis and compared the image quality. RESULTS: At half the radiation dose, digital flat-panel X-ray detector radiography provided equivalent image quality of the liver and spleen, lumbar vertebrae 2 and 5, pelvis, and psoas margin on abdominal radiographs. The image quality obtained with digital flat-panel X-ray detector radiography of the kidneys, the hollow cavities of the upper efferent urinary tract, and the urinary bladder was judged to be statistically better than those obtained with computed radiography. CONCLUSION: With half the exposure dose of computed radiography, the flat-panel X-ray detector produced urograms with an image quality equivalent to or better than computed radiography.

[Endovascular gamma-irradiation for prevention or restenosis after angioplasty of femoropopliteal de-novo-stenoses-long-term results of a feasibility study]. / Die endovaskuläre gamma-Bestrahlung zur Prävention der Restenose nach Perkutaner Transluminaler Angioplastie von de-novo-Stenosen femoropoplitealer Arterien-Langzeitergebnisse einer Pilotstudie.

OBJECTIVES: To evaluate the performance and efficacy of endovascular irradiation after percutaneous transluminal angioplasty (PTA) of de-novo femoropopliteal stenoses in a pilot study. METHODS: 6 patients received non-centered endovascular irradiation (12 Gray at surface of the vessel wall) immediately after angioplasty of de-novo femoropopliteal stenosis, 1 patient was given centered endovascular irradiation using 192-iridium (12 Gray at surface of the vessel wall) Centered irradiation was considered for two other patients. Duplex sonographies and interviews were performed the day before and after PTA and after 1, 3, 6, 9, 12, 18, 24 months up to 4 years. Intraarterial angiography was performed in symptomatic patients. RESULTS: Non-centered endovascular irradiation was possible in all patients without problems or complications. Centered irradiation was not possible in two patients with the cross-over approach. One thromboembolic complication occurred during centered irradiation. Both restenosis and new stenosis at the edge of irradiated distance occurred in 1/7 patients. No other side effects were observed during follow-up. CONCLUSIONS: In our pilot study endovascular irradiation after angioplasty of de-novo femoropopliteal stenosis was possible with low rates of complications and restenosis and taking vessel anatomy into account.

Functional magnetic resonance imaging of human pontine auditory pathway.

The purpose of this study is to visualize brainstem auditory pathways by functional magnetic resonance imaging (fMRI). Eighteen healthy volunteers (age 28 to 42 years) with normal hearing function underwent fMRI examination on a 1.5 Tesla imaging system (Philips, Best, The Netherlands) with periodic click stimulation. Blood oxygen level dependent images were obtained using a three-dimensional EPI sequence with shifted echo technique (principles of echo shifting with a train of observations). Control scans without click stimulation were obtained in the identical setting. Cross correlation activation maps were calculated using a postprocessing tool (Philips). They were matched with anatomic slices of identical orientation and thickness. Five of 18 subjects were excluded because of motion artifacts. In 4/13 significant activation was observed at the root entry zone of the ipsilateral acoustic nerve corresponding to the cochlear nuclei. In 11/13 subjects, significant activation was found in the same slice contralaterally close to the floor of the 4th ventricle, corresponding to the expected region of the superior olivary nucleus. Activation of the rostral parts of the auditory pathway (inferior colliculus, medial geniculate body) was not found. In the absence of the stimulus no activation occurred in these structures. It was concluded that activation of the brainstem auditory pathways by click stimuli can be visualized by fMRI.

Ultrasound examination of carotid and vertebral arteries.

Ultrasound is the most widespread diagnostic procedure in obstructive disease of the arteries supplying the brain. The combined non-invasive information on morphology and function makes duplex ultrasound the procedure of choice in screening and follow-up of carotid artery disease. This review deals with all relevant aspects of color duplex ultrasound of the carotids and the vertebral arteries. After a short introduction into the clinical background, the paper focuses on aspects of examination technique. In the main part of the review the relevant ultrasound findings in carotid artery disease are discussed. The different methods for grading stenoses of the internal carotid artery are explained in detail. Other relevant pathologies, such as vertebral artery disease, dissection and aneurysms, are briefly mentioned. The clinical value of ultrasound in the work-up of carotid and vertebral artery disease is briefly discussed in comparison with other imaging procedures.

Age related signal decrease in functional magnetic resonance imaging during motor stimulation in humans.

Right handed healthy volunteers underwent functional magnetic resonance imaging (fMRI) examinations on a 1.5 Tesla MRI-scanner (Gyroscan ACS NT; Philips, Best, NL). Blood oxygen level dependent (BOLD) images were obtained using a three dimensional multi-shot echo planar imaging sequence employing a shifted echo technique (Principles of echo shifting with a train of observations). Finger tapping of the right hand was used as a task for motor stimulation. A total of 86 subjects was included into statistical analysis. Absolute and relative signal differences and cluster sizes of activation for the left motor cortex were obtained. In addition, Z-score, pooled Z-score and cross correlation activation maps were calculated and matched with high resolution anatomic images. A significant decrease with age could be detected for absolute and relative signal intensity differences for the whole group and for the male subgroup. Correlation analysis for the female subgroup also bore negative albeit non-significant correlation coefficients. An age-related decline of BOLD-contrast can be assumed to explain signal decrease. This age-related effect should be considered in clinical fMRI applications.

Age- and sex-specific differences in the 1H-spectrum of vertebral bone marrow.

Correct interpretation of 1H magnetic resonance (MR) studies of the red vertebral bone marrow in patients with disorders of the hematopoietic system requires knowledge of the dependence of the proton spectrum on age and sex. Infiltration of malignant cells causes a decreased red bone marrow fat signal, which is reversed upon successful treatment. The aim of this study was to establish a database of normal water/fat relations from a large group of volunteers. In all, 154 healthy volunteers from 11 to 95 years of age were examined using a 1.5-T MR system (ACS-NT, Philips). A volume of 2-8 ml in the center of a normal vertebral body was selected for spectroscopy using the PRESS sequence without water suppression (TR/TE 2 sec/40 msec). Signal intensities and T2 times of lipid and water resonances were determined. The relative fat signal intensity was calculated corrected for T2 relaxation. The relative proportion of protons in the fat signal increases with age from 24% in the age group 11-20 years to 54% in the group > or = 61 years. The proportion of fat in the vertebral bone marrow in female subjects is less than that in male subjects in the same age group (statistically significant at P < or = 0.01). In the central age group between 31 and 50 years, the difference is largest, at 12%; in the youngest and oldest age group this difference is small. T2 times are neither age nor sex dependent.

[Proton-MR spectroscopy of the spinal bone marrow. An analysis of physiological signal behavior]. / Protonen-MR-Spektroskopie des Wirbelkörpermarks. Analyse des physiologischen Signalverhaltens.

BACKGROUND: Magnetic resonance imaging has shown to be a sensitive method for diagnostics of the red bone marrow, the composition of which changes physiologically and during pathological processes. However, the interpretation of MRI in patients with disorders of the red bone marrow is very difficult. The aim of this study was the characterization of the proton spectrum of healthy bone marrow and its age- and sex-dependent changes to obtain a data basis for measurements in patients. METHODS: 154 healthy volunteers have been examined. After imaging, a spectroscopic measurement was performed to determine the relative intensities of fat and water, and their respective T2 times. RESULTS: While T2 (water: 46.9 ms and fat: 75.4 ms) does not depend on age or sex, the relative signal intensity of fat increases by about 6% per decade. In the age groups between 31 and 50 years it diverges significantly between men (43.5%) and woman (32.5%) (p < or = 0.01, Mann-Whitney-Test. CONCLUSIONS: Proton spectroscopy can increase the reliability of diagnosis by offering information on composition of the marrow. The analysis of spectroscopic measurements requires exact knowledge about normal physiological values.

[Indications and rate of repeat studies in the color duplex department of a radiology university clinic]. / Zuweiser und Rate der Doppeluntersuchungen in der farbduplexsonographischen Abteilung einer radiologischen Universitätsklinik.

PURPOSE: Who refers patients for a color duplex ultrasonography (CDUS) in the routine work of a radiological university department and how frequent are repeat examinations? MATERIAL AND METHODS: 1110 patients had at least one CDUS examination between 5/97 and 5/98. The specialities of the referring physicians/primary examiners and the inpatient or outpatient status of the patients were documented at the first consultation. Additionally the patients were asked by the medical staff, whether, how often, and by whom ultrasonographic, CT, or MR examinations of the same organ system had been carried out during the last 4 weeks without evidence of any clinical changes. RESULTS: 97% of the 1118 patients were referred by one of the university departments. The arterial system was exclusively investigated in 58% and the venous system in 29% of the cases. 81% of the 651 arterial examinations were requested by the Departments of Vascular/Cardiac Surgery and Radiology. 75 repeated ultrasonographic examinations were documented in 67 (6%) of the 1118 consultations. Repetitions were documented in 8% of the arterial and in 3% of the venous examinations. X-ray angiographies were already done or planned in 105 of the 625 arterial CDUS (16%). Ultrasonographic referrals with parallel phlebographies (1% of 320 venous indications) as well as parallel CT and MR examinations (1% of the 1118 consultations, respectively) were the exception. CONCLUSIONS: In the described setting, CDUS was mainly used to assess the arterial vascular system prior to vascular surgery and radiological interventions. Repeat ultrasonographies alone (6%) and parallel examinations altogether (14%) were observed less frequently than expected.

[Precision of MRI-guided needle placement--experimental results]. / Präzision der Lokalisierbarkeit einer Punktionsnadel in der MRT--experimentelle Ergebnisse.

PURPOSE: To estimate the accuracy of the determination of tip position of MR compatible biopsy needles based on signal voids and artifacts in MR guided biopsies. METHOD: In a nylon grid phantom an MR compatible 18G needle (E-Z-EM) was measured in a 1.0 T imager using TSE- and FFE-sequences of 20 s and 40 s duration in 34 different orientations of needle versus B0, frequency and slice selection gradient. 4 radiologists with no experience in the evaluation of signal void artifacts estimated the needle tip positions from needle tip artifacts. The readers determined the needle tip before and after a 15 minute training session based on high resolution images with explanation of size and shape of specific artifacts of biopsy needles in 12 different orthogonal or parallel orientations to B0 and frequency encoding gradient that are possible if the needle lies parallel to the slice, i.e. orthogonal to the slice selection gradient. The values obtained before and after the training session were compared to the real position of the needle tip. RESULTS: Mean distance of actual needle tip and tip position as determined from images was 1.8 +/- 2.3 mm in TSE-versus 2.5 +/- 1.2 mm in FFE-images, with the needle length overestimated. After a 15 minute training session the positioning error decreased significantly to 0.2 +/- 1.8 mm for TSE-sequences and to 1.0 +/- 1.8 mm for FFE-sequences. A higher accuracy of tip localization was obtained with TSE sequences. CONCLUSION: In MR guided biopsies using FFE- and TSE-sequences the needle position can be more accurately determined if the reader is familiar with the 12 orthogonal or parallel positions of the needle with respect to B0 and frequency encoding gradient and the corresponding artifacts.

MR imaging of the chest: mediastinum and chest wall.

This review presents the options and limitations of MRI in non-vascular diseases of the mediastinum and the chest wall. In numerous thoracic pathologies, MRI is a useful supplement to spiral CT. This imaging procedure also allows a contrast-media-free differentiation of solid tumors and vascular lesions (e. g., aortic aneurysms). The advantages of MRI over CT are particularly useful when multiplanar tumor imaging is required prior to surgery to establish the exact spatial relationship between tumor and the other mediastinal structures. Primary indications for MRI in diseases of the mediastinum and chest wall are therefore: (a) tumors of the posterior mediastinum for determining their position in relation to the neural foramina and the spinal canal; (b) chest wall tumors; (c) preoperative multiplanar imaging of primary mediastinal tumors; and (d) contraindications against CT exams with iodine contrast media.

[Frequency of repeated examinations in the ultrasonographic section of a radiological university department]. / Häufigkeit von Doppeluntersuchungen in der B-Bild-sonographischen Abteilung einer radiologischen Universitätsklinik.

PURPOSE: What is the percentage of repeat examinations in the ultrasonographic section of a radiological university department? METHODS: All 4116 patients, who were first examined in the sonographic section of the Radiologic Department of the University of Cologne from 5/97 to 5/98, were asked by the medical staff, whether, how often and by whom ultrasonographic, CT or MR examinations of the same organ system had been carried out during the last 4 weeks without evidence of clinical changes. The data were documented using a structured questionnaire. RESULTS: Sonographic repeated examinations documented in 443 (10.8%) of the 4116 consultations. The proportion of in- to outpatients with repeat examinations was 26.0% to 74.0%. Contrarily, the repeated ultrasonographies were done by practitioners in 62.3% and by the university staff in 33.2% (radiology 48.2%). With regard to the 4116 consultations, the abdomen (81.1%) was the organ system most often examined repeatedly. Computed tomographies were already done or planned in 305 of 4116 patients (7.4%) and MR-tomographies were already done or planned in 57 patients (1.4%). CONCLUSION: The rate of repeated ultrasonographic examinations in a radiological university hospital was 19.6%.